Seems tough to ensure only subwoofer is playing bass frequencies (& not the main speakers)

The integrated’s subwoofer/mono output is actually a speaker level output. The RCA (right/left) LFE input on the subwoofer is a line level input. Don’t think its possible to hook them up (if I understand you correctly). My integrated does not have pre-outs. Using pre-outs wont help as the full range signal still goes to the main speakers.

that sub has ’output high pass’

https://www.velodyneacoustics.com/digital-drive-plus-series/

the sub grabs the bass, and crosses over at either 80hz or 100hz as you choose.

you send full range separate left and right signals to sub first, then it sends separate left and right signals without the low bass back to the preamp/integrated amp/tape loop, lots of options, then to your mains.

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Self-powered Subs: two advantages

1. amp does not need to amplify bass, so a lower power amp can be used, an important consideration regarding cost, weight, heat, and especially allows you to try less powerful tube amps.

2. mains do not try to produce bass that they can only do 'moderately', so you get the main’s mids and highs mixed with the sub’s bass capability.

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2 channel system, sub always on presumably, low bass always to the self powered sub, high pass to mains.

in addition to above, my old velodyne also allows speaker wires from the amp, thru it’s variable equalizer, then speaker wires without the low bass to the speakers. mains drivers do not have to try and do bass they cannot do well. no advantage to the amp, it still gets full signal from the preamp, amplify s the bass prior to sub.

I am in favor of a stereo pair of subs, forward firing, located next to the mains, producing primary bass notes and all the overtones of those notes, thus the bass is directional and improves imaging, i.e. where is the jazz bassist standing?

also, I am no fan of ports, passive radiators, .... again, to preserve bass imaging
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multi-channel/home theater, a single self powered sub is often enough, just to add the dinosaur stomps, black hawk down heliocopters ...

you want your full range signal going to the mains, then, in the av receiver, you tell it what size mains you have. multi-channel it equalizes what it sends hither and thither, however

often 2 channel, or direct sounds better than multi-channel, without the sub. In those cases you want your mains to receive all the bass, and do the best they can without the sub.

In this case, the passive woofers from the ported main speakers produce qualitatively different bass than the high powered sealed subwoofer. I'd prefer to let 1 or more subs deal with bass... and let the mains deal with the rest.

A tough sell. aka stubborn. They ALL produce qualitatively different bass. The exact same sub produces qualitatively different bass simply by being put in a different place in the room. Did you not look at my system? Three different types of subs.  

You can lead a horse to water....

roxio, yes you understood me correctly. Looking at the Primaluna manuals somewhat poor back panel image I mistook that subwoofer output to be an RCA, my bad.

 elliottbnewcombjr is correct. The plus does have XLR and RCA Inputs that can be switched to route the signal through a high pass crossover. 
The manual doesn't mention that flexibility using the High Level Inputs.

Nor does it mention the use or result of a single high (speaker) level connection. I'm hesitant to suggest connecting one high (speaker) level cable from the integrated to any of the +/- subwoofers inputs since the manual shows only all four high level (speaker) cables connected.

Possibly someone can comment on using a second set of speaker cables coming from the integrated amplifiers speaker outputs to the Velodyne's High Level Inputs (page 17 Velodyne User's Manual)?

Unless the Integrated amplifier is having a noticeable undesirable affect driving your main speakers to there potential there may little or no advantage by using a high pass filter.
In my opinion the ability to use the Velodyne's on board Auto/Manual Equalization, Room Optimization, and Remote settings, would be far more desirable.

Consider the effort it would take to have a proper set of XLR or RCA outputs installed on the Integrated amplifier instead of a single high level output.
The Velodyne Plus possesses outstandingly useful abilities not found in most other subwoofer despite its design being being over a decade old. The Frequency Response and Parameters Screen (page 10,11 Velodyne User Interface Manual) is a huge advancement that should be made familiar to its user. 

High level sub inputs use a resistor to drop the voltage down to line level. Because of this the extra connections present no load and the amplifier can drive them just fine. If the OP would study my system, which is highly recommended, he would see there is even a picture showing this exact resistor modded inside my amp to turn it into a line out to drive my subs. 

Exactly what he wants to do. Only minus the unnecessary and counterproductive crossover stuff he's so determined to mess up with.